Long-exposure NuSTAR constraints on decaying dark matter in the Galactic halo

We present two complementary NuSTAR x-ray searches for keV-scale dark matter decaying to monoenergetic photons in the Milky Way halo. In the first, we utilize the known intensity pattern of unfocused stray light across the detector planes---the dominant source of photons from diffuse sources---to separate astrophysical emission from internal instrument backgrounds using $\ensuremath{\sim}7$- Ms/detector deep blank-sky exposures. In the second, we present an updated parametric model of the full NuSTAR instrument background, allowing us to leverage the statistical power of an independent $\ensuremath{\sim}20$-Ms/detector stacked exposures spread across the sky. Finding no evidence of anomalous x-ray lines using either method, we set limits on the active-sterile mixing angle ${\mathrm{sin}}^{2}(2\ensuremath{\theta})$ for sterile-neutrino masses 6--40 keV. The first key result is that we strongly disfavor a $\ensuremath{\sim}7$-keV sterile neutrino decaying into a 3.5-keV photon. The second is that we derive leading limits on sterile neutrinos with masses $\ensuremath{\sim}15--18\text{ }\text{ }\mathrm{keV}$ and $\ensuremath{\sim}25--40\text{ }\text{ }\mathrm{keV}$, reaching or extending below the big bang nucleosynthesis limit. In combination with previous results, the parameter space for the neutrino minimal standard model is now nearly closed.